A certain number of conventional devices already exist, which are applicable to the measurement "in line" of the flow rate of a mixed flow in a conduit of a gaseous phase and a liquid phase. The devices of this type nevertheless do not provide really satisfactory results except in the particular cases where there is only little or no slip between the two phases (low flow rates of one of the two phases, quasi homogeneous flows). On the other hand, in the case where the volumetic gas-liquid ratio becomes greater than a given threshold, considerable slip may occur between the two phases, which may give rise to nonhomogeneous flows of various types: stratified, annular, plug flows, etc. . . . In such cases, none of these conventional devices is any longer able to ensure the required precision.
To overcome these drawbacks due to slip between phases, one often resorts, when for example the flow rate of an oil well is to be evaluated, to another technique, which consists in provisionally installing a separator on the outlet conduit of the well and in measuring individually the flow rates of the phases. The principal drawback of this method is that it requires cumbersome and heavy apparatures (measurement made at high pressure), the transport of which may be difficult at certain work sites. In addition, the separators presently used for this type of measurement are not in general designed for an installation in line and thus are not suitable for a continuous surveillance of the flow rate of an individual well.
In order to remedy the different drawbacks mentioned above, there has been recently proposed by the applicant (see U.S. Pat. No. 4,056,002 incorporated by reference in the present patent application) a new process for measuring the flow rate of each of the components of a two-phase flow. This process isolates, at a determined point of the conduit which conveys the gas-liquid two-phase flow of which one wants to measure the flow rate, successive fractions of this flow, so as to temporarily "freeze" during the time of measurement the slip between phases of this flow, and to cause the volume of each of the fractions thus isolated to vary, so as to turn to account the great difference in compressibility which exists between the gas and the liquid constituents of the said flow to deduce therefrom the respective proportions of liquid and of gas enclosed in each of these fractions, counting the fractions isolated per unit time thus permitting the flow rate of each of the phases of this flow to be deduced. Such a process has the major advantage over other processes known to date of allowing the measurement of flow rates of two-phase flows having significant slip between the phases, without it being necessary to resort to a separation of these phases. It follows that the different apparatuses which may be envisaged for carrying out this process may consequently assume a particularly compact form, while these apparatuses may moreover be fixedly installed or on the contrary be utilized as apparatuses for sporadic control.
Among the different apparatuses which may be envisaged for carrying out this process, there have been essentially described in the above-mentioned U.S. Pat. No. 4,056,002, apparatuses in which volumetric devices of the "vane pump" type are used to ensure the division of the composite flow into separate, successive fractions and the variation of the volume of these successive fractions. The utilization of these devices of the "vane pump" type creates certain problems in cases where one must proceed to measure two-phase flows liable to contain in addition solid materials in suspension (for example sand, pebbles . . . ), such as oil flows. It has indeed been indicated in the U.S. Pat. No. 4,056,002 that the utilization of these volumetric devices of the "vane pump" type was not obligatory and that one could likewise envisage utilizing devices of other types, such as devices of the "eccentric screw pump" kind commercially known by the name of Moyno pumps. There has nevertheless not been described in this U.S. Patent any exact embodiment of such "eccentric screw pumps". This utilization of eccentric screw pumps with a view to carrying out the process of flow-rate measurement defined above, could nevertheless not be immediately deduced from the embodiments relating to the "vane pumps", given that it seems a priori nearly impossible to conceive "eccentric-screw pumps" wherein the volume of the cells could be varied in a continuous manner as in the "vane pumps".